This is a known bug in the YUI editor. SInce YUI 2 isn't supported anymore I've fixed it myself. If you update the file www/js/yui/editor/editor.js in ariadne from the latest SVN, this bug should be gone.
For the curious, cleanHTML() - which is called when switching to source code view - uses regular expressions to translate certain tags into other tags. This is generally a bad idea, but especially if you don't check for word boundaries on tags... the fix I made just added the word boundary '\b' after each tag name so that it will only match the exact tag.
regards,
Auke van Slooten
Muze